After the Louisiana Purchase in 1803, the United States sought knowledge of the vast territory west of the Mississippi River by sending explorers to study the land and the native peoples who lived there. Their observations in words, maps, and pictures built on one another and gradually put Minnesota on the map. The first U.S. expedition west from Ft. Snelling was led by Major Stephen H. Long in 1823 through the Minnesota and Red River valleys with scientists, a landscape painter, and an interpreter. In 1835, English geologist G.W. Featherstonhough further explored the region, kept detailed journals, and later published an important eyewitness account of a frontier in transition as traders, missionaries, and the military gradually forced the Dakota from their tribal lands and traditional way of life. In 1836, artist and author George Catlin recorded the Pipestone Quarry in what became southwestern Minnesota, including the religious rites of Indians quarrying stone at the sacred site for carving and trading across native North America. Commissioned by the U.S. Army in 1838, French map-maker Joseph Nicollet and his assistant John Fremont led two surveying expeditions into the land between the Missouri and Mississippi rivers, and Nicollet's extraordinarily accurate map became a lasting achievement of western exploration.